Sandwich structures are used extensively in aerospace, automotive, and marine applications. Standard sandwich structures include a foam core and outer layers, called face sheets, that are adhesively bonded to the foam core. The face sheets of some sandwich structures are composites formed of fibrous materials, such as fiberglass or graphite, impregnated with a suitable resin. In a number of sandwich structure applications, the face sheets function to carry the applied loads, and the foam core transfers loads from one face sheet to the other. Other sandwich structures are configured so that the foam core also absorbs a portion of the applied loads. In either case, it is important that all layers maintain their connection to one another.
Maintaining the connections of the layers, particularly of the face sheets to the foam core during applied loading, is problematic. The most common source of face sheet separation stems from the relatively weak adhesive hold between the face sheets and the foam core, as compared to the applied loads. That is, the pulloff strength is low. Efforts to solve this problem in the past have focused on improving the adhesive that bonds the face sheets to the foam core.
Another source of layer separation is due to differences in the coefficient of thermal expansion (CTE) of the different materials used to form the layers. As a result, as temperatures rise or fall, the material used to form one layer may expand or contract more quickly than the material used to form another, adjoining layer. In addition to causing layer separation, CTE differences can significantly distort the shape of a structure, making it difficult to maintain overall dimensional stability. Current efforts to solve this problem focus on more closely matching the CTE of one layer to that of its adjoining layers.
A second issue in current sandwich structure design concerns how to optimize the thickness of a structure to meet the weight and/or space limitations of its proposed application. Sandwich structures are desirable because they are usually lighter in weight than solid metal counterparts, but they can be undesirable in that they usually require more space. Moreover, in some instances, it is necessary to pass an object (e.g., wires or tubing) through a structure. While this is relatively easy to accomplish in a solid metal structure by cutting holes in the structure, this is more difficult to accomplish in a sandwich structure. Simply carving out a portion of a layer of a sandwich structure can undesirably reduce the load carrying capability of the overall structure, as well as complicate the manufacture of the structure.
U.S. Pat. No. 5,186,776 describes a technique for reinforcement of composite laminates utilizing an apparatus and method for heating and softening the laminates by ultrasonic energy, penetrating the laminate, moving the laminate fibers aside, inserting a reinforcing fiber into the laminate and allowing the laminate and fiber to cool and bond. The technique disclosed in U.S. Pat. No. 5,186,776 is hereby incorporated by reference.
U.S. Pat. No. 4,808,461 describes a structure for localized reinforcement of composite structure including a body of thermally decomposable material that has substantially opposed surfaces, a plurality of reinforcing elements in the body that extend generally perpendicular to one body surface, and pressure intensifying structure on the other opposed body surface for applying driving force to the reinforcing elements for insertion into the composite structure as the body is subjected to elevated temperature and decomposes. The technique disclosed in U.S. Pat. No. 4,808,461 is hereby incorporated by reference.
U.S. Pat. No. 5,589,016 and U.S. Pat. No. 5,624,728 describe a planar composite panel constructed from two resin-impregnated fiber face sheet coverings and bonded to the two sides of a honeycomb core element, and a surrounding border element made of rigid foam board. The two planar faces of the rigid foam board are embossed with a pattern of indentations in the form of interlinked equilateral triangles which are sufficiently deep and close together to provide escape paths for volatiles generated inside the panel during curing of the resin in the face sheets by which the face sheets are bonded to the honeycomb core element and the foam board, to prevent the development of excessive pressure between the face sheets that otherwise would interfere with the bonding. The techniques disclosed in U.S. Pat. No. 5,589,016 and U.S. Pat. No. 5,624,728 are hereby incorporated by reference.
Thus, a need exists for a method of forming a foam core sandwich structure that resists distortion and separation between layers, in particular, separation of the face sheets from the foam core; maintains high structural integrity; resists crack propagation; and easily accommodates the removal of portions of foam core, as required by specific applications. The method should allow the structure to be easily manufactured and formed into a variety of shapes. The present invention is directed to providing such a method and the resulting structure. The present invention also relates to column core structural materials where the foam is removed but where the pins remain.